Turbochargers are well known and widely used with internal combustion engines. Generally, turbochargers supply more charge air for the combustion process than can otherwise be induced through natural aspiration. This increased air supply allows more fuel to be burned, thereby increasing power and torque obtainable from an engine having a given displacement. Additional benefits include the possibility of using lower-displacement, lighter engines with corresponding lower total vehicle weight to reduce fuel consumption, and use of available production engines to achieve improved performance characteristics. Some turbocharger applications include the incorporation of an intercooler for removing heat (both an ambient heat component and heat generated during charge air compression) from the charge air before it enters the engine, thereby providing an even more dense air charge to be delivered to the engine cylinders. Intercooled turbocharging applied to diesel engines has been known to at least double the power output of a given engine size, in comparison with naturally aspirated diesel engines of the same engine displacement.
Additional advantages of turbocharging include improvements in thermal efficiency through the use of some energy of the exhaust gas stream that would otherwise be lost to the environment, and the maintenance of sea level power ratings up to high altitudes.
At medium to high engine speeds, there is an abundance of energy in the engine exhaust gas stream and, over this operating speed range, the turbocharger is capable of supplying the engine cylinders with all the air needed for efficient combustion and maximum power and torque output for a given engine construction. In certain applications, however, an exhaust stream waste gate is needed to bleed off excess energy in the engine exhaust stream before it enters the turbocharger turbine to prevent the engine from being overcharged. Typically, the waste gate is set to open at a pressure below which undesirable predetonation or an unacceptably high internal engine cylinder pressure may be generated.
At low engine speeds, such as idle speed, however, there is disproportionately little energy in the exhaust stream as may be found at higher engine speeds, and this energy deficiency prevents the turbocharger from providing a significant level of boost in the engine intake air system. As a result, when the throttle is opened for the purpose of accelerating the engine from low speeds, such as idle speed, there is a measurable time lag and corresponding performance delay, before the exhaust gas energy level rises sufficiently to accelerate the turbocharger rotor and provide the compression of intake air needed for improved engine performance. The performance effect of this time lag may be pronounced in smaller output engines which have a relatively small amount of power and torque available before the turbocharger comes up to speed and provides the desired compression. Various efforts have been made to address this issue of time lag, including reductions of inertia of turbocharger rotors.
In spite of evolutionary design changes for minimizing the inertia of the turbocharger rotor, however, the time lag period is still present to a significant degree, especially in turbochargers for use with highly rated engines intended for powering a variety of on-highway and off-highway equipment.
Furthermore, to reduce exhaust smoke and emissions during acceleration periods when an optimal fuel burn is more difficult to achieve and maintain as compared with steady-speed operation, commercial engines employ devices in the fuel system to limit the fuel delivered to the engine cylinders until a sufficiently high boost level can be provided by the turbocharger. These devices reduce excessive smoking, but the limited fuel delivery rate causes a sluggishness in the response of the engine to speed and load changes.
The turbo-lag period can be mitigated and, in many instances, virtually eliminated by using an external power source to assist the turbocharger in responding to engine speed and load increases. One such method is to use an external electrical energy supply, such as energy stored in d.c. batteries to power an electric motor attached to the turbocharger rotating assembly. The electric motor can be external and attached to the turbocharger rotor through a clutching mechanism, or it can be added onto the turbocharger rotating assembly and energized and de-energized through the use of appropriate electronic controls.
Turbocharging systems with integral assisting motors are more completely described in our pending U.S. patent application Ser. No. 08/529,672.
Other patents disclosing turbocharger-electrical machine combinations include U.S. Pat. Nos. 5,406,797; 5,038,566; 4,958,708; 4,958,497; 4,901,530; 4,894,991; 4,882,905; 4,878,317 and 4,850,193. More particularly, U.S. Patent No. 5,406,797 discloses turbocharger with a rotary electric machine which is added to the rotatable shaft of a turbocharger. Power for operation of the rotary electric machine is provided by an a.c. electric generator held in engagement with the flywheel of the engine. An inverter converts the a.c. electric power produced by the electric generator to an a.c. output having a predetermined frequency, the inverter including a rectifier for rectify the generated a.c. electric power to d.c. electric power, and yet another power device is required for converting the d.c. electric power to a.c. electric power having the predetermined frequency to energize the rotary electric machine.
U.S. Pat. No. 4,958,708 discloses another rotary electric machine which is added to the turbocharger shaft to which the intake air compressor and exhaust turbine are affixed. The rotary electric machine of this patent comprises a rotor composed of permanent magnets and a stator with polyphase windings driven from an invertor/controller.
The attachment of the permanent magnets to the turbocharger shaft has a major disadvantage in that the magnets are subjected to heat which is conducted along the shaft from the hot turbine wheel of the turbocharger. This can present a significant problem in that the permeability of the magnets may be reduced by such heating to a level which may be unacceptable for efficient operation of the rotary electric machine. This becomes a serious problem when the turbocharged engine is subjected to a hot shutdown and the oil flow through the bearings and over the shaft is interrupted. A steep temperature gradient will exist for a significant length of time while the hot parts of the turbocharger are drained of their heat content.
Notwithstanding the efforts to develop motor-assisted turbocharger systems, there is still a need for an improved turbocharger which improves the performance and low-speed response characteristics of the conventional internal combustion engine and for an improved motor-assisted supercharging apparatus, such as motor-driven compressor.